Article pubs.acs.org/crystal
ZnMoO4 Micro- and Nanostructures Synthesized by Electrochemistry-Assisted Laser Ablation in Liquids and Their Optical Properties Y. Liang, P. Liu, H. B. Li, and G. W. Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Nanotechnology Research Center, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, People's Republic of China ABSTRACT: Electrochemistry-assisted laser ablation in liquids (ECLAL) is a chemically “simple and clean” synthesis of nanoparticles. Using ECLAL, we have synthesized zinc molybdate nanoplates and nanorods with two different phases. These two nanostructures are characterized carefully by scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, Fourier transform infrared spectroscopy, Raman scattering spectroscopy, and UV−vis spectrophotometry. On the basis of the cathodoluminescence measurements, we observe that the nanoplates emit no light, while the nanorods give out green light before annealing. The optical properties of both get much better after annealing, which indicates their potential applications in photoelectric nanodevices. The basic physics and chemistry involved in the ECLAL fabrication and the luminescence mechanism for products before and after annealing are discussed.
thermal methods,16,17 electrospinning calcination combinations,18 or solid state reactions19 with the above obvious flaws. In this contribution, we synthesize mass zinc molybdate nanoplates and nanorods with two different phases at one time and observe different optical performance of the as-synthesized samples before annealing. Interestingly, these nanostructures broke down and crystallized, respectively, and exhibit much better optical performance after annealing, which indicates their potential applications in photoelectric nanodevices. These investigations have further demonstrated that the ECLAL method is a general strategy for the fabrication of simple POM nanostructures and also an effective method for the fabrication of rare mineral hydrates, which are hard to synthesize by ordinary chemical methods.
1. INTRODUCTION Polyoxometalates (POMs) represent a diverse range of microand nanoclusters based upon oxides of at least binary metal containing Mo, W, V, or Ta and a second transition metal such as Cu, Fe, Co, or Ni species. Such cluster building blocks can be self-organized in complex framework which leads to functional materials with diverse properties such as optical, electronic, magnetic, and catalytic activity.1 To improve the performance of POMs, several techniques for the synthesis of POM nanostructures have been developed in recent years. However, these techniques have many obvious flaws, for example, hightemperature or high-pressure environment, various templates or additives, and demanding complicated synthetic procedures causing impurities in final products, which greatly restrict their application in industry. In this study, combining laser ablation in liquids (LAL)2−8 and electrochemistry, we develop a facile synthesis for simple POM nanostructures, that is, electrochemistry-assisted laser ablation in liquids (ECLAL). This chemically “simple and clean” technique is performed in an ambient environment without extreme temperature and pressure and can be designed by combining interesting solid targets, electrodes, and mother solutions.9,10 Herein, for the fabrication of zinc molybdate nanostructures, we chose molybdenum as a solid target for laser ablation, zinc electrodes for the electrochemical reaction, and deionized water as a solvent for the ECLAL synthesis. As a typical transition metal material, zinc molybdates, commonly applied as luminescent materials,11,12 croyogenic phonon-scintillation detectors,13 or catalysts,14 have roused the attention of scientists in recent years. They are mainly synthesized by precipitation heat-treatment methods,15 hydro© 2012 American Chemical Society
2. EXPERIMENTAL PROCEDURES 2.1. Materials Preparation. The detailed the experimental setup of ECLAL synthesis has been reported in our previous works.9,10 In this case, a single crystalline Mo target with 99.95% purity is used as the starting material and is initially attached to the bottom of the rectangular quartz chamber with dimensions 6.0 × 3.0 × 6.5 cm3. Deionized water (highly pure water) with resistivity of 16.8−17.2 MΩ·cm was then poured slowly into the chamber until the target was covered by 2−3 mm. Meanwhile, a DC electrical field with an adjusted voltage of 160 V, which is brought by two parallel electrodes with a distance of 50 mm, is applied above the target. The second harmonic is produced by a Q-switch Nd:YAG laser device with a wavelength of 532 nm, a pulse width of 10 ns, a repeating frequency of 5 Hz, and a Received: May 15, 2012 Revised: July 29, 2012 Published: August 7, 2012 4487
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Figure 1. SEM images of the synthesized coexisting zinc molybdates (a), micro- and nanoplates (b), and rods (c) and their corresponding images after annealing (d, e). pulse energy of 150 mJ/pulse. Finally, the pulsed laser was focused onto the surface of the Mo target. During ablation, the target and liquid environment were both maintained at room temperature. After the whole intersection, which lasted for 30 min, the solution was evaporated and collected for further measurements. To further investigate the properties of zinc molybdates, the as-synthesized samples were annealed at 500 °C in high vacuum furnace for 2 h. 2.2. Characterization Techniques. Scanning electron microscopy (SEM) images of the as-synthesized samples were obtained by a JSM-7600F field emission scanning electron microscope operated at 15 kV. Before observation, the samples were covered by a gold layer to decrease the charge effect during the analysis. X-ray diffraction (XRD) was performed with a Rigaku D/Max-IIIA X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å, 40 kV, 20 mA) at a scanning rate of 1° s−1, and transmission electron microscopy (TEM) was carried out with a JEOL JEM-2010HR instrument at an accelerating voltage of 200 kV, equipped with an energy-dispersive X-ray spectrometer (EDS). These techniques are used to identify the crystal structure and morphology of the products. Inductively coupled plasma-atomic (ICP) emission spectrometry using a ThermoFisher iCAP6500Duo has been employed to analyze the chemical content of starting materials, with an incident power of 1150 W, a plasma gas flow of 14 L/min, and an atomization gas flow of 0.6 L/min. To gain molecular vibration
information, Raman spectra (RS) and Fourier transformation infrared (FTIR) spectra of samples deposited on Si substrate were recorded on a Renishaw inVia Plus laser micro-Raman spectrometer detected with Ar ion laser irradiation of λ = 514.5 nm and Bruker EQUINOX55 spectrometer coupled with infrared microscope, respectively. UV−vis spectrum of the obtained liquid sample was also obtained by a UV 3150 spectrophotometer (Shimadzu, Japan). Moreover, cathodoluminescence (CL) spectroscopy with a Gatan MonoCL3 system attached to a field emission scanning electron microscope was employed to characterize the luminescence of the synthesized nanostructures at room temperature.
3. RESULTS AND DISCUSSION 3.1. Morphology and Structure Characterization. Figure 1 shows the morphology of the as-synthesized sample. The products consist of homogeneous micro- and nanoplates and rods under the same experiment conditions. The corresponding high-magnification image demonstrates that the nanoplates are actually structured as a flower-like shape with a perfect three-dimensional geometry, about 2−3 μm in diameter, 100 nm in thickness, and nanoscaled surface smoothness, as shown in Figure 1b. Meanwhile, the high4488
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are more likely to be easily hydrated Zn5Mo2O11·5H2O and nanorods are likely to be unhydrous ZnMoO4 since they behave differently in heating process. In other words, the Zn5Mo2O11·5H2O may decompose during the thermal treatment while ZnMoO4 remain the same at 500 °C. Figure 3a,e shows the TEM bright-field images of a single nanoplate and rod gently transferred onto a copper grid by ultrasonication. HRTEM examination indicates that the nanoplate is of single crystal structure without visible defects, as shown in Figure 3b. This HRTEM image clearly shows the interplanar spacing, measured as 0.272 and 0.268 nm, in good agreement with the d values of the (22̅1) and (202) crystallographic planes of the rhombohedral Zn5Mo2O11·5H2O. Figure 3c gives the corresponding selected area electron diffraction (SAED) pattern, which further proves the singlecrystal nature of nanoplates. By calculation, the diffraction point can be indexed to (22̅1), (202), and (021) facet of Zn5Mo2O11·5H2O. Similarly, HRTEM examination of a single rod shows that the nanorod is of single-crystal structure. The fringe pattern is not clear-edged and regularly arranged as highcrystallinity crystals are, which indicates that the nanorods are not well crystallized. The planar spacing, measured as about 0.667 and 0.631 nm in Figure 3e, is very close to (1̅10) and (01̅1) facets of triclinic ZnMoO4. Its corresponding Fourier transform SAED analysis in Figure 3f further proves the poor crystallinity of nanorods since the diffraction point is broadened and is identified as (1̅10) (01̅1) and (1̅01) facets of ZnMoO4. Calculation shows that the (1̅ 2̅ 3 ) crystal planes are perpendicular to the axis of the nanorods, indicating that ⟨1̅2̅3⟩ is the preferred growth direction of ZnMoO4. A reasonable explanation is that the relatively low free energy of growth of the (1̅2̅3) crystal plane leads to the ⟨1̅2̅3⟩ direction being preferable. The EDS results, performed on HRTEM, are illustrated in Figure 4. In addition to the carbon and copper signal from the carbon-coated copper grid, the products are composed of Zn, Mo, and O elements. Therefore, the TEM results assuredly confirm that the synthesized nanoplates are hydrate Zn5Mo2O11·5H2O and the rods are anhydrate ZnMoO4, which corresponds to the SEM and XRD results. Quantitative correlation between Zn5Mo2O11·5H2O and ZnMoO4 has been obtained using the ICP technique. The mass concentration of Zn and Mo elements is determined to be 16.01 and 7.02 mg/L; thus, the Moore percentage of Zn/Mo under the same volume is about 3:2; therefore we can infer the relative content of Zn5Mo2O11·5H2O and ZnMoO4 is about 4:1, which means the output of nanorods is far less than the nanoplates in our experiment. And it is not too hard to understand that the XRD results only contain diffraction peaks of the Zn5Mo2O11·5H2O before annealing. Now let us discuss the nucleation mechanism of zinc molybdates nanocrystal synthesis upon ECLAL. The general formation mechanism of nanostructures upon laser ablation in liquid has been addressed in our groups’ previous work.8 When laser ablates the solid Mo target, Mo ions and other species plasma plume will be generated at the surface between the liquid and solid under high temperature, high pressure, and high density conditions, and H+, OH−, and O2− species results from the deionized water at the same time. Mo species can easily combine with O 2− species to form the initial molybdenum oxide (MoO3 or even MoO42− ions) microand nanostructure nuclei. Meanwhile, the applied electric field is suggested to play an important role in the formation mechanism of nanostructures by ECLAL.9,10 An electro-
magnification image of the nanorods (Figure 1c) shows a tendency of oriented growth, about 100 nm in diameter and several micrometers in length. Careful observation reveals several nanoscaled holes at the ends of the rods, which means their crystallinity is not sufficient. These samples are annealed at 500 °C in a high vacuum furnace for 2 h, and the corresponding SEM images show different results for nanoplates and rods. Figure 1d reveals several micro- or nanoholes formed on the surface and inner parts of the plates, which indicates their decomposition. In contrast, the surface of nanorods become smoother with clear edges and corners shown in Figure 1e, which indicates improved crystallinity. The corresponding XRD patterns for unannealed sample are illustrated in Figure 2a. All peaks can be indexed to
Figure 2. The corresponding XRD pattern of the products obtained before (a) and after (b) anealing. The colored lines at the bottom correspond to the standard XRD patterns of different zinc molybdates.
rhombohedral zinc molybdenum oxide hydrate Zn5Mo2O11·5H2O (JCPDS Card File No. 30-1486), except one peak for Si substrate as shown. Only the highest peak is assigned to be the (003) plane of Zn5Mo2O11·5H2O. The crystalline structure of annealed samples is also performed as shown in Figure 2b. Only one peak marked with a blue line can be indexed to triclinic ZnMoO4 (JCPDS Card File No. 350765), and the other three peaks are assigned as Au (JCPDS Card File No.65-2870), which results from the spraying process. SEM and XRD results demonstrate that the nanoplates 4489
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Figure 3. TEM bright field images of a single zinc molybdate micro- and nanoplates and rods (a, d). The corresponding SAED patterns (b, e) and HRTEM images (c, f).
due to lowering of the nucleation free energy barrier.20 So in our case, the nucleation mechanism of zinc molybdates is most likely to be fast self-organized heterogeneous germination in the tridimensional space. When the crystal nucleus forms, the Zn5Mo2O11 ·5H2O and ZnMoO4 begin to form large particles. Two factors may be responsible for the formation of these nanoplates and nanorods. On the one hand, different kinds of chemical reactions have taken place in the liquid environment, which offers crystal seeds of Zn5Mo2O11·5H2O and ZnMoO4 nanostructures for growth. On the other hand, the applied electric field can provide extra electrostatic energy and induce the formation of different morphologies based on the theory of crystalline habits by weakening of the surface energies. The TEM result, which shows that the high index ⟨1̅2̅3⟩ is the preferred growth direction of ZnMoO 4 , indicates the importance of extra electrostatic energy during its growth process. As is well-known, laser ablation in liquid is a very fast process and far from equilibrium, meanwhile the electric field is very strong, which can be estimated as 2.7 × 103 V/m. Thus, the whole process is very effective under the high-temperature,
chemical reaction happened: Zn2+ ions easily dissolve into the liquid environment under strong applied electric field and form Zn(OH)2 in the presence of H and O ions, which are also electrolyzed by the applied field. The reaction equation is Zn + 2H 2O → Zn(OH)2 + H 2
(1)
Then, a chemical reaction takes place between Zn species and Mo species. These two highly active species can easily crash into each other during the plasma quenches under the strong electric field, which leads to the formation of zinc molybdate metastable clusters. 2MoO3 + 5Zn(OH)2 → Zn5Mo2O11·5H 2O
(2)
MoO4 2 − + Zn 2 + → ZnMoO4
(3)
The first step for crystal nucleation is often facilitated by heterogeneous centers.20,21 These foreign bodies range from solid or liquid particles to the wall of the container and even the clusters of metaphase of nucleating materials. The generally accepted mechanism of heterogeneous nucleation is that it follows the kinetic law for homogeneous nucleation but is faster 4490
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Figure 4. FTIR spectrum (a), Raman spectrum (b), and UV−vis absorption spectrum (c) of the as-synthesized zinc molybdates. Panel d is the typical Tauc plot derived from the UV−vis absorption.
895−950 and 810−880 cm−1.27,28 As we can see, bands at 913 and 866 cm−1 for nanoplates and bands at 950 and 860 cm−1 for nanorods can be assigned to symmetric stretching (ν1) and asymmetric stretching (ν 3 ) vibration modes of MoO 4 tetrahedral units of zinc molybdates.28 The Raman band observed at 815 cm−1 for nanorods may result from a stretching vibration in Mo−O bonds because of the distorted MoO4 tetrahedral units. The Raman band at 345 cm−1 for nanorods and 309 cm−1 for nanoplates can be assigned to the symmetric bending vibration (ν2) or asymmetric bending vibration (ν4), which usually lies in the region of 300−520 cm−1.28,29 Therefore, FTIR and Raman data further show that the synthesized nanostructures are zinc molybdates. 3.3. UV−Vis Absorption. UV−vis spectroscopy is employed for understanding the optical properties of the assynthesized zinc molybdates, and the data are shown in Figure 4c. Clearly, in the UV spectrum, there are two absorption bands located at 235−250 and 210−220 nmy, which indicate two different phases of the as-synthesized zinc molybdates corresponding to the previous results. Typically, the optical bandgap (Eg) of semiconductor materials can be estimated by the classical Tauc approach,30,31 which presents the relationship between the incident photoenergy (hν) and the absorption coefficient (α) near the absorption edge, as follows
high-density, and high- pressure (HTHPHD) state and high external electric field. In addition, suitable experiment parameters play an important role in the formation of these nanostructures. 3.2. FTIR and Raman Spectra. The chemical structure information of the zinc molybdates is further identified by the FTIR spectrum. Figure 4a shows the corresponding result in the range of 400−4000 cm−1. Very few papers in the literature have reported the FTIR spectrum of Zn5Mo2O11·5H2O and ZnMoO4, and our analysis is based on molybdates. Several absorption bands of samples are present; generally, the infrared bands at 3324 and 1650 cm−1 correspond to the OH stretching vibration and bending vibration of water molecules.22 Since we have measured the blank sample for comparative analysis, the bands at 1106 and 613 cm−1 are due to the SiO2 substrate.23 Other bands that are located in the range of 700−1000 cm−1 wavenumbers are mainly caused by [MoOy]n−.24,25 Bands at 1509, 1396, 2922, and 2850 cm−1 may be attributed to the organic contamination of the obtained samples during the sample preparation. Finally, the band at 473 cm−1 is most likely to be Zn−O stretching mode in zinc molybdates.26 Raman scattering spectra of zinc molybdate nanoplates and nanorods in the wavelength range 50−1200 cm−1 are also displayed in Figure 4b. Usually, the symmetric stretching modes (ν1) and asymmetric stretching modes (ν3) of MoO4 tetrahedral units are Raman active and observed in the region
αhν = A 0(hν − Eg )n 4491
(4)
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The value of the exponent depends on the mechanism of interband transition (for example, n = 1/2 for direct transitions and n = 2 for indirect transitions). A0 is a constant called the band tailing parameter, and Eg is the intercept of the extrapolated linear when (αhν)1/n is plotted against hν. Figure 4d shows an example of a typical Tauc plot. In our case, the band gap value of indirect transition for zinc molybdates is estimated to be 4.48 and 4.72 eV from the above plots, respectively, and 4.48 eV is close to the theoretical value of the band gap of triclinic ZnMoO4 as reported.32 The band gap of Zn5Mo2O11·5H2O nanoplates is determined to be 4.72 eV for the first time by our experiment. 3.4. Luminescence Performance of Zinc Molybdates. We measured the cathodoluminescence of the as-synthesized samples at room temperature, and the CL spectra are shown in Figure 5. It was found that these zinc molybdate nanostructures display different properties when annealed at 500 °C in high vacuum furnace for 2 h. Specifically, for nanoplates, there are two peaks after annealing, one broad emission band centered at
370 nm and another emission band centered at 510 nm, while no peak shows before annealing. For nanorods, there is only one emission band centered at 530 nm before and after heat treatment; nevertheless, the luminous intensity of annealed nanorods increased almost 30 times compared with unannealed ones. The inset in Figure 5a,b shows the SEM images and corresponding panchromatic CL images for unannealed nanoplates and nanorods, while those in Figure 5c,d are images for the annealed ones. By comparison of the panchromatic CL images, both nanostructures also exhibited much better luminescence performance after annealing. Now we discuss the whole luminescence mechanism of samples during the process. Luminescent properties of triclinic ZnMoO4 small single crystals and bulk crystals have been reported only by a few papers.32−36 It was found that zinc molybdates belong to the class of self-activated phosphors, and the intrinsic emission of ZnMoO4 originates from the radiative annihilation of self-trapped excitons on MoO42− complexes, similarly to other self-activated tungstates and molybdates of AXO4.32,37 The emission peak at 530 nm for nanorods in our experiment is close to the experimental result of 544 nm for bulk ZnMoO4 crystals by X-ray excitation at room temperature.35 For annealed nanoplates, CL spectra show an obvious peak at 370 nm, which is close to 380 nm (3.26 eV) and can be attributed to the near band edge emission.38 Therefore, we infer that the nanoplates broke down into other phases, such as ZnO and ZnMoO4, while the nanorods crystallized during the heating process, which can take place as follows: Zn5Mo2O11·5H 2O → 2ZnMoO4 + 3ZnO + 5H 2O
(5)
The reaction equations are confirmed by XRD results of samples annealed as shown in Figure 2. In our case, ZnO is not detected because of its inhomogeneity and low content. Based on above analysis, we can have a clear and general insight into the basic mechanisms involved in the whole process: The nonluminous Zn5Mo2O11·5H2O nanoplates broke down into luminous ZnO (emission peak at 370 nm) and ZnMoO4 (emission peak at 510 nm) after annealing, while the greenemitting ZnMoO4 nanorods (peak at 530 nm) crystallized and the luminous intensity improved several times. The peak shift may result from imperfections of various origins (defects, inadvertent impurities during the crystal growth process, or compound decomposition processes) and the difference between nanometer materials and bulk ones.32 Overall, the whole luminescent process for zinc molybdate nanoplates and rods are controllable, which indicates their potential applications in photoelectric nanodevices such as light-operated switches.
4. CONCLUSION In summary, the coexisting zinc molybdate micro- and nanoplates and nanorods have been successfully synthesized upon ECLAL. It was found that the as-synthesized zinc molybdate nanostructures are identified as nonluminous Zn5Mo2O11·5H2O nanoplates and green-emitting ZnMoO4 nanorods, and the luminescence properties of both nanostructures become much better after annealing. The controllability of the fabrication indicated their potential applications in photoelectric nanodevices. Our investigations have further demonstrated that ECLAL is a general strategy for fabricating simple POMs with specific properties and also an effective method for the fabrication of rare mineral hydrates, which are hard to synthesize by other means.
Figure 5. The room-temperature CL spectrum obtained from zinc molybdate micro- and nanoplates and rods. The insets in each graph: (a) SEM images of several plates or rods before annealing; (b) corresponding panchromatic CL images; (c) SEM images of a single plate or several rods after annealing; (d) corresponding panchromatic CL images. 4492
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(29) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1963; p 107. (30) Wood, D. L.; Tauc, J. Phys. Rev. B 1972, 5, 3144−2151. (31) Tauc, J. Amorphous and Liquid Semiconductors; Plenum Press: London, 1974; Chapter 4. (32) Spassky, D. A.; Vasil’ev1, A. N.; Kamenskikh, I. A.; Mikhailin, V. V.; Savon, A. E.; Hizhnyi, Y. A.; Nedilko, S. G.; Lykov, P. A. J. Phys.: Condens. Matter 2011, 23, No. 365501. (33) Mikhailik, V. B.; Kraus, H.; Wahl, D.; Ehrenberg, H.; Mykhaylyk, M. S. Nucl. Instrum. Methods Phys. Res. A 2006, 562, 513−516. (34) Nagornaya, L. L. IEEE Trans. Nucl. Sci. 2009, 56, 2513−2518. (35) Gironi, L.; Arnaboldi, C.; Beeman, J. W.; Cremonesi, O.; Danevich, F. A.; Degoda, V. Y.; Ivleva, L. I.; Nagornaya, L. L.; Pavan, M.; Pessina, G.; Pirro, S.; Tretyak, V. I.; Tupitsyna, I. A. J. Instrum. 2010, 5, 11007−11019. (36) Spassky, D. A.; Vasil’eva, A. N.; Kamenskikh, I. A.; Mikhailin, V. V.; Savon, A. E.; Ivleva, L.; Voronina, I.; Berezovskaya, L. Phys. Status Solidi A 2009, 206, 1579−1583. (37) Zhang, Y.; Holzwarth, N. A. W.; William, T. R. Phys. Rev. B. 1998, 57, 12738−12750. (38) Yang, Y. H.; Chen, X. Y.; Feng, Y.; Yang, G. W. Nano Lett. 2007, 7, 3879−3883.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS NSFC (Grants U0734004 and 11004253) and the Ministry of Education supported this work.
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